1. Field of the Invention
The present invention is directed to piezoelectric injection systems having a mechanism for controlling rate shape, and to methods for controlling such piezoelectric injection systems.
2. Description of Related Art
In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to inject fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring so that when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of the injection event.
In another type of system, such as disclosed in U.S. Pat. No. 5,819,704, the beginning of injection event is controlled by a servo-controlled needle valve element. The system includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. Specifically, it is known that improved control of fuel metering into the combustion chamber, is essential in reducing the level of emissions generated during combustion process while minimizing fuel consumption, for example, in combustion of diesel fuel. In addition, it is known that improved control of the rate of fuel injected during the course of an injection event, i.e. the rate shape of the injection, is also very important in reducing the level of emissions generated, especially in diesel fuel combustion. As a result, many proposals have been made to provide fuel metering control and rate shape control for closed nozzle fuel injector systems, including such systems that utilize piezoelectric fuel injectors.
For instance, U.S. Pat. No. 5,779,149 to Hayes, Jr. discloses a piezoelectric controlled common rail fuel injector. The piezoelectric actuator controls the movement of an inwardly opening poppet-type control valve for controlling the flow of fuel from a control volume and ultimately, the movement of the nozzle valve element. The reference further discloses that fuel metering is variably controlled by controlling the duration and modulation of the electrical signal that is provided to the piezoelectric actuator. Although the above-described reference provides some control over fuel metering, and thus, control over the amount of fuel injected, the reference does not provide a solution for effectively controlling rate shape of the fuel injections.
U.S. Pat. No. 6,253,736 to Crofts et al. discloses a piezoelectric fuel injector nozzle assembly having feedback control with a nozzle valve control arrangement that operates to control the movement of the nozzle valve element. The reference discloses that the nozzle valve control arrangement functions to control the quantity of the fuel metered, and also functions as a rate shaping control device for producing a predetermined time varying change in the flow rate of fuel injected into the combustion chamber during an injection event so as to improve combustion and minimize emissions. The reference further discloses that the injection rate shape is controlled by varying the voltage supplied to the piezoelectric actuator based on engine operating conditions.
U.S. Pat. No. 4,732,129 to Takigawa et al. discloses an injector with an electroexpansive actuator. The actuator voltage is controlled to ultimately vary the movement of a nozzle needle thereby enabling fuel injection at different injection rates.
U.S. Pat. No. 6,367,453 to Igashira et al. discloses a method of controlling injection rate shape by applying voltage to piezo actuator such that the injection rate increases slowly when voltage is applied to the piezo actuator and decreases rapidly when voltage is applied to the piezo actuator is stopped, thereby creating a triangular rate shape. The injector uses a three-way valve and a specific size ratio of main and sub orifices to achieve slow needle opening and quick needle closing motion.
Methods of controlling fuel injectors such as that disclosed in Crofts et al. typically provide an input signal, i.e. voltage, current, etc., to a piezoelectric element, an electromagnetic actuator, or a magnetostrictive actuator to thereby operate the fuel injector. As disclosed in Crofts et al., rate shape of fuel injections is also controlled in the same manner by changing the magnitude of the input signal. However, controlling the rate shape of fuel injections by varying the input signal in the manner known has been found to not provide the desired results in various instances when accurate rate shaping would be desirable.
Thus, despite the teachings of the art discussed above, alternative systems and methods for controlling injection rate shape using piezoelectric fuel injectors are desirable to provide further control of combustion and emissions generated by such combustion, and to further improve fuel economy. Therefore, there still exists an unfulfilled need for a piezoelectric fuel injection system having enhanced rate shape control, and a method for controlling a piezoelectric fuel injector in which enhanced rate shape is attained.